nth_value and row_number in a partition

Hi,

I can't find the right syntax to use simultaneously nth_value and row_number
in a partition.

I have a table with roads and their orders, a road segment name and two
values v1 and v2

I want to decrement first_value v1 of each partition with the current record
value v2 of the partition in a third column v3 in the same record. So I
thought that I could combine and row_number which returns "number of the
current row within its partition, counting from 1" with nth_value

My problem is that I get either syntax error near over, window function
needs an over clause, nth_value(integer, bigint) doesn't exists. Query works
if I just uses something like

nth_value(v2,1) but I can't find how I can replace "1" with the current row
position in the partition (1,2,3…)

I tried dozen of syntax with no success. Here is one of them

select roads,orders,(first_value(v1) over (partition by roads,segment order
by orders)-(nth_value(v2,(cast(row_number() as integer) over (partition by
roads,segments order by orders))) over (partition by roads,segments order by
orders))) as result from my table

Thanks for any help !

Olivier